Drilling fluids or mud are utilized when drilling a wellbore through rock formations in order to carry the rock cuttings created at the bit up to the surface where they are removed. To control downhole pressures, the fluid's density is usually increased by the addition of weight agents such as barite. The fluid should therefore exhibit sufficient viscosity to provide efficient cuttings removal (hole-cleaning), and sufficient gel strength for the stable suspension of barite. Drilling mud or fluids should also exhibit a low filtration rate (Fluid Loss) in order to maintain mud rheology and characteristics.
During the product manufacturing, storage and transportation, O2 may migrate and dissolve into the products. At HPHT extreme condition O2 will decompose and form oxygen free radical. Free radical may trigger polymer chain scission at the weak links along polymer backbone that leads to polymer degradation: polymer will quickly lose its properties such as molecular weights, mechanical strength, rheology and viscosity. In HTHP oilfield drilling application, the degradation of polymer will cause detrimental effects on the drilling mud properties, such as loss in mud viscosity and rheological characteristics. It is critical to solve these problems and maintain polymer initial thermal stability/performance during HT application. Antioxidants can interact with free radicals and terminate the chain scissions, therefore protect polymers from O2 and thermal induced degradation and maintain their initial attributes.
During oilfield application, some metal oxides for example, iron oxides may be introduced into the drilling muds from all possible sources, e.g. weighting agents, pipes and drilling equipments. The contamination of metal oxides will likely trigger the polymer degradation. Chelating agents can be introduced into the mud formulation, which can sequestrate the metal impurities and prevent polymer decomposition from happening, therefore protect polymer and maintain drilling mud characteristics.
U.S. Pat. No. 7,384,892 assigned to Hercules Incorporated discloses a water-based drilling fluid composition including water and at least one rheology modifier and/or fluid loss control agent, and at least one other ingredient of polymeric additive, inorganic salts, dispersants, shale stabilizers, weighting agents, or finely divided clay particles, depending upon the desired attributes, wherein the rheology modifier and/or the fluid loss control agent comprises carboxymethylated raw cotton linters (CM-RCL) made from the baled raw cotton linters or comminuted raw cotton linters with increased bulk density.
An unpublished U.S. Provisional Application No. 61/638,142 dated Apr. 25, 2012 by International Specialty Products discloses compositions containing a synergistic combination with high fluid loss control and excellent retention of rheological properties even under HPHT conditions. In accordance with certain aspects, the present application is directed to a dispersion polymerization process and chemistry modification to make fluid loss control and rheological modifier polymers. Moreover, the fluid loss additives and rheology modifiers disclosed herein can be delivered to the field as water dispersions or dry powders, either separately or blended together, to facilitate handling and processing.
An unpublished U.S. Provisional Application No. 61/610,203 dated Mar. 13, 2012 by International Specialty Products discloses a high pressure, high temperature (HPHT) fluid loss additive and, in particular, to a fluid loss additive for oil-field drilling applications. In accordance with one aspect of the invention, the fluid loss additive comprises a terpolymer of acrylamide (AM), 2-acrylamido-2-methyl-propanesulfonic acid (AMPS) and a cationic monomer such as acrylamidopropyl-trimethyl ammonium chloride (APTAC) and/or methacrylamidopropyltrimethyl ammonium chloride (MAPTAC). In accordance with particularly useful embodiments, the polymer composition comprises from about 30-70 wt. % acrylamide, 30-70 wt. % AMPS, and 2-50 wt. % APTAC and/or MAPTAC. AMPS can be in either acid or neutralized form used in the polymerization process.
It has been very challenging to retain original properties of rheology modifiers and fluid loss additives of drilling mud composition and there has arisen a need for improved performance of water-based drilling mud/fluid composition comprising synthetic polymeric components that exhibits improved tolerance to high temperatures, high pressure, and metal impurities.
Surprisingly, we have found that the combination of antioxidants and chelating agents provided “double protection” for the polymers in the HPHT oilfield application.
Accordingly, it is an object of the present application to provide a drilling mud composition which is HPHT stable and capable of preventing degradation of polymer based components such as Fluid Loss Additives (FLA) and Rheology Modifiers (RM) that are essential part of drilling mud composition.